NATURAL CARBON SINKS & GLOBAL WARMING

A carbon dioxide sink is a carbon reservoir that is increasing in size, and is the opposite of a carbon "source". The main natural sinks are (1) the oceans and (2) plants and other organisms that use photosynthesis to remove carbon from the atmosphere by incorporating it into biomass. This concept of carbon dioxide sinks has become more widely known because of its role in the Kyoto Protocol.

Carbon sequestration is the term describing processes that remove carbon from the atmosphere. To help mitigate global warming, a variety of means of artificially capturing and storing carbon — as well as of enhancing natural sequestration processes— are being explored.

Forests and the carbon cycle

Carbon is incorporated into forests and forest soils by trees and other plants. Through photosynthesis, plants absorb carbon dioxide from the atmosphere, store the carbon in sugars, starch and cellulose, and release the oxygen into the atmosphere. A young forest, composed of growing trees, absorbs carbon dioxide and acts as a sink. Mature forests, made up of a mix of various aged trees as well as dead and decaying matter, may be carbon neutral above ground. In the soil, however, the gradual buildup of slowly decaying organic material will continue to accumulate carbon, but at a slower rate than an immature forest. The forest eco-system may eventually become carbon neutral. Forest fires release absorbed carbon back into the atmosphere.

The dead trees, plants, and moss in peat bogs undergo slow anaerobic decomposition below the surface of the bog. This process is slow enough that in many cases the bog grows rapidly and fixes more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs inter approximately one-quarter of the carbon stored in land plants and soils.

Under some conditions, forests and peat bogs may become sources of carbon dioxide, such as when a forest is flooded by the construction of a hydroelectric dam. Unless the forests and peat are harvested before flooding, the rotting vegetation is a source of carbon dioxide and methane comparable in magnitude to the amount of carbon released by a fossil-fuel powered plant of equivalent power.

Forests as a carbon sink

Forests are carbon stores, and they are carbon dioxide sinks when they are increasing in density or area. Thus, reforestation can mitigate global warming until all available land has been reforested with mature forests]. In the United States in 2004, forests sequestered 10.6% as much carbon dioxide as was released in the United States by the combustion of fossil fuels. Urban trees sequestered another 1.5%. To further reduce U.S. carbon dioxide emissions by 7%, as stipulated by the Kyoto Protocol, would require the planting of "an area the size of Texas [8% of the area of Brazil] every 30 years", according to William H. Schlesinger, dean of the Nicholas School of the Environment and Earth Sciences at Duke University, in Durham, N.C.. Carbon offset programs are planting millions of fast-growing trees per year to reforest tropical lands, for as little as $0.10 per tree; over their typical 40-year lifetime, one million of these trees will fix 0.9 teragrams of carbon dioxide.

The global cooling effect of forests is partially counterbalanced: For example, the planting of new forests may initially be a source of carbon dioxide emission when carbon from the soil is released into the atmosphere. Also, reforestation can decrease the reflection of sunlight (albedo): Mid-to-high latitude forests have a much lower albedo during snow seasons than flat ground, thus contributing to warming.

A long-term sequestration of carbon from forests comes from the use of wood products such as "stick built" (i.e., with lumber) homebuilding, the predominant form of home building in the US. Because most buildings are eventually demolished, the carbon may be released into the atmosphere, depending upon the fate of the scrap lumber. Reusing the lumber, or using it as fuel to replace a fossil fuel, avoids an increase in atmospheric carbon. In addition to global cooling, planting forests reduces erosion, increases water capture, and provides valuable timber which may be sustainably harvested.

soils as a carbon sink

Carbon as plant organic matter is sequestered in soils: Soils contain more carbon than is contained in vegetation and the atmosphere combined. Soils' organic carbon (humus) levels in many agricultural areas have been severely depleted. Grasslands contribute to soil organic matter, mostly in the form of roots, and much of this organic matter can remain unoxidized for long periods.

Since the 1850s, a large proportion of the world's grasslands have been tilled and converted to croplands, allowing the rapid oxidation of large quantities of soil organic carbon. However, in the United States in 2004 (the most recent year for which EPA statistics are available), agricultural soils including pastureland sequestered 0.8% as much carbon as was released in the United States by the combustion of fossil fuels. The annual amount of this sequestration has been gradually increasing since 1998.

Methods that significantly enhance carbon sequestration in soil include no-till farming, residue mulching, cover cropping, and crop rotation, all of which are more widely used in organic farming than in conventional farming. Because only 5% of US farmland currently uses no-till and residue mulching, there is a large potential for carbon sequestration. Conversion to pastureland, particularly with good management of grazing, can sequester even more carbon in the soil. Terra preta, an anthropogenic, high-carbon soil, is also being investigated as a sequestration mechanism.

oceans as a carbon sink

Oceans are natural carbon dioxide sinks, and represent the largest active carbon sink on Earth. This role as a sink for carbon dioxide is driven by two processes, the solubility pump and the biological pump. The former is primarily a function of differential carbon dioxide solubility in seawater and the thermohaline circulation, while the latter is the sum of a series of biological processes that transport carbon (in organic and inorganic forms) from the surface euphotic zone to the ocean's interior. A small fraction of the organic carbon transported by the biological pump to the sea floor is buried in anoxic conditions under sediments and ultimately forms fossil fuels such as oil and natural gas.

At the present time, approximately one third of anthropogenic emissions are estimated to be entering the ocean. The solubility pump is the primary mechanism driving this, with the biological pump playing a negligible role. This stems from the limitation of the biological pump by ambient light and nutrients required by the phytoplankton that ultimately drive it. Total inorganic carbon is not believed to limit primary production in the oceans, so its increasing availability in the ocean does not directly affect production (the situation on land is different, since enhanced atmospheric levels of carbon dioxide essentially "fertilize" land plant growth). However, ocean acidification by invading anthropogenic carbon dioxide may affect the biological pump by negatively impacting calcifying organisms such as coccolithophores, foraminiferans and pteropods. Climate change may also affect the biological pump in the future by warming and stratifying the surface ocean, thus reducing the supply of limiting nutrients to surface waters.

One way to increase the carbon sequestration efficiency of the oceans is to add micrometer-sized iron particles called hematite or iron sulfate to the water. This has the effect of stimulating growth of plankton. Iron is an important nutrient for phytoplankton, usually made available via upwelling along the continental shelves, inflows from rivers and streams, as well as deposition of dust suspended in the atmosphere. Natural sources of ocean iron have been declining in recent decades, contributing to an overall decline in ocean productivity (NASA, 2003). Yet in the presence of iron nutrients plankton populations quickly grow, or 'bloom', expanding the base of biomass productivity throughout the region and removing significant quantities of carbon dioxide from the atmosphere via photosynthesis.

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